1. Field of the Invention
The present invention relates to a protective structure for semiconductor sensors, and more particularly a monolithic MIS protective structure for semiconductor sensors.
2. Description of the Related Art
Sensors that are exposed to environmental influences in order to be able to detect measured quantities cannot in any case be protected from electric or electrostatic overstress as effectively as other semiconductor devices. Ion-sensitive field-effect transistors (ISFETs), for example, do not have a metallic gate at which protective structures could be realized. On the contrary, here the gate insulator lies functionally exposed in the channel area of the transistor and is thus vulnerable to electric damaging. Such negative influences are, for example, electrostatic discharges (ESD) or disallowed electrical operating conditions such as excess voltage, polarity inversion and the like.
Ion-sensitive field-effect transistors are used in the analysis of liquid media, for example. In these sensors, for example, a drain-source current is regulated through ions in a measuring medium having direct contact with the gate insulator and causing, depending on the ion charge, an electric field there that influences the drain-source current whereby the measuring medium can be analyzed. Therefore, these sensors are also referred to as field-effect based sensors. For a flawless mode of operation and for the stability of the gate insulator, it is important that these semiconductor sensors are protected from a discharge of static voltages and/or from disallowed electrical operating conditions such as polarity inversions.
Currently, voltage-sensitive sensor structures can be protected by allocating a structure to the sensor element that can incorporate, for example, p-n junctions or also capacitor elements. A substantial disadvantage of protective structures in the form of conventional p-n junctions is the relatively high leakage current, whereby a precise potentiometric measurement can be influenced negatively. On the other hand, capacitors are disadvantageous as protective structures to the effect that they are conducting for alternating currents (e.g. EMC stress) and do not provide effective protection from other disallowed operating cases (polarity inversion).